The present invention relates to a method and a circuitry for demodulating a digital frequency-modulated signal to improve the signal within the small signal range.
Many methods are known for digitally demodulating a frequency-modulated (FM) signal, for example the CORDIAL algorithm. These demodulators present the problem that the signal-noise ratio decreases disproportionately at the demodulator output when the signal-noise ratio of the input signal falls below a certain threshold. This threshold is normally called the FM threshold. This critical point is reached approximately when the noise amplitude is of the same order of magnitude as the signal amplitude. Phase jumps then occur in the demodulated signal.
It is therefore an object of the present invention to create a method and a circuitry for digitally demodulating a frequency-modulated signal, with which signal the aforementioned drawbacks are largely avoided and the FM reception is improved.
The object is achieved in that, after demodulating the digitized frequency-modulated signal for generating the signal""s amplitude A(n) and differential phase value xcfx86xe2x80x2(n), the following steps are performed:
comparing the amplitude A(n) of the demodulated signal with a constant K at determined sampling times n,
multiplying the differential phase value xcfx86(n) of each sampling time n by factor c, factor c being the product of amplitude A(n) of the determined sampling time and amplitude A(nxe2x88x921) of a preceding sampling time, if amplitude A(n) is less than constant K.
According to the present invention, therefore, instantaneous amplitude value A(n) is additionally provided at the demodulator output and the frequency-modulated signal is modified by this amplitude value. Signal correction occurs only when the signal amplitude drops below a specific value, so that the pulse spikes are reduced in the signal. It has been established that a preceding amplitude value is also advantageously considered in the signal correction and that the demodulator output signal to be corrected, namely differential phase xcfx86(n), is multiplied by the product of amplitude value A(n) and preceding amplitude value A(nxe2x88x921).
When amplitude value A(n) is greater than or equal to chosen constant K, then the factor should be equal to 1, so that the demodulator output signal remains unchanged.
Advantageously, the method should be performed continuously, amplitude A(n) being compared with constant K and the demodulator output signal being multiplied with the appropriate factor c at each sampling time n. It is advisable that preceding amplitude value A(nxe2x88x921) to be considered be amplitude value A(nxe2x88x921) directly preceding instantaneous amplitude value A(n).
In particular, the method is suitable for FM receivers, the following steps being performed before demodulating the digital frequency-modulated signal:
converting an analog high-frequency signal into a digital signal,
mixing the digital signal into a base band,
filtering the base band signal.
The circuitry according to the present invention correspondingly has
a time delay element at the output of the demodulator for the digital amplitude value A(n),
a comparator at the output of the demodulator and time delay element for comparing instantaneous amplitude value A(n) with constant K,
a multiplier for forming the product of instantaneous amplitude value A(n) and preceding amplitude value A(nxe2x88x921), factor c being equal to the product, if instantaneous amplitude value A(n) is less than constant K,
a multiplier for multiplying differential phase signal xcfx86xe2x80x2(n) by factor c, for generating a demodulated output signals MAX.
Advantageously, the time delay element is designed so that the product is formed from instantaneous amplitude value A(n) and directly preceding amplitude value A(nxe2x88x921).
When instantaneous amplitude value A(n) is greater than or equal to constant K, factor c should be set equal to 1.